Plaster composition containing water-reducing agent

ABSTRACT

A plaster composition containing an agent for reducing the amount of water necessary in forming a pourable aqueous calcined gypsum slurry, the agent comprising a high potassium salt of a condensation product of naphthalene and sulfonic acid having a molecular weight between about 300 and 3,000. Aqueous plaster slurries containing the agent are pourable, moldable and castable with much less excess water than ordinarily required, resulting in dried cast products of increased strength and other physical property benefits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the process of a setting calcinedgypsum slurry, and is more particularly concerned with a composition forreducing the amount of water necessary in forming a pourable aqueouscalcined gypsum slurry to use in making cast or molded products.

2. Prior Art

Plaster (i.e. calcined gypsum) has long been a large volume commercialarticle of commerce either as various dry plaster compositions to bemixed by the user with water in forming cast gypsum articles or inmanufactured products such as gypsum wallboard and cast art objects andthe like. Generally, molds or cast gypsum articles are manufactured bydispersing calcined gypsum and additives in sufficient water to form apourable slurry, casting the slurry into a mold of desired shape,allowing the slurry to set, and drying excess water from the setarticle.

The gypsum setting reaction involves the reaction of calcium sulfatehemihydrate and water to form calcium sulfate dihydrate. The theoreticalwater required to convert the calcined gypsum to set gypsum dihydrate isonly 18.7% by weight on a pure basis. However, considerable excess wateris required to fluidize the calcined gypsum and obtain proper flow ofthe gypsum slurry in the casting or molding operation. The amount ofexcess water depends primarily upon the type of plaster particle,whether beta hemihydrate or alpha hemihydrate, and other additives inthe plaster composition. Additives conventionally used in minor amountsinclude accelerators, retarders, fibrous reinforcements, and consistencyreducers. Consistency reducing agents are typified by thelignosulfonates, gum arabic, modified starches and other cellulosicderivatives. Ordinarily, plasters produced at different locations fromdifferent sources will also vary in their water requirements to form apourable slurry. Thus, calcined gypsum compositions that arepredominantly or substantially beta hemihydrate may have a normalconsistency in a range of about 60-90 or more cc. This relates to awater usage of 60-90 cubic centimeters of water per 100 grams of thecalcined gypsum in order to form a readily pourable and flowable gypsumslurry. Other calcined gypsum compositions that predominantly containalpha hemihydrate may have a normal consistency of 34-55 cc. forexample.

The use of cellulosic additives as consistency reducers in calcinedgypsum plasters has been suggested as early as around 1900.

In the area of another type of inorganic cementitious material, Portlandcement, various other materials have been suggested as additives tomodify properties of the composition. For example U.S. Pat. No.3,465,825 suggests the use of the mixed salts of lithium and sodium ofthe condensation products of mononapthalene sulfonic acid andformaldehyde as fluid loss control agents in Portland cementcompositions. Such compositions are highly alkaline, generally pH 11 andhigher, and the use of such additives have been thought to be effectiveonly in highly alkaline media. For certain specific industrialapplications it has been possible to combine substantial amounts ofPortland cement compositions and plaster compositions. See for exampleU.S. Pat. Nos. 3,582,376; 3,847,635 and 3,852,081. Such compositionsbecause of their high alkalinity due to the substantial quantities ofthe Portland cementitious ingredients have taught the utilization ofnaphthalene sulfonic acid and formaldehyde condensation productconsistency reducing additives. However, the available materials of thistype have not been entirely effective or satisfactory in non-highlyalkaline media and their usage has not been recommended in plastercompositions which have not been adjusted to high alkalinity. Inaddition attempts to utilize commercially available condensates of thistype in plaster compositions has revealed a severe detrimentalcharacteristic of the condensate. Compositions containing substantialamounts of Portland cements are generally a dark grey in color; however,predominantly plaster containing compositions, especially those mostusable in the manufacture of pottery and artware objects are very lightin color and present a quite pleasing appearance. The addition ofcommercially available sodium form of naphthalene sulfonic acid andformaldehyde condensates resulted in a brown discoloration developing inthe cast article on exposure to sunlight and with time, whichdiscoloration is highly objectionable from a marketing standpoint.

SUMMARY OF THE INVENTION

It has now been found that a highly effective plaster consistencyreducing additive which does not cause objectionable cast discolorationmay be prepared from commercially available naphthalene sulfonic acidand condensation products by controlled treatment with a basic compoundcontaining potassium such as potassium hydroxide or potassium sulfate.

Accordingly, it is a principal object and advantage of the presentinvention to provide a plaster consistency reducer which is highlyeffective in decreasing the amount of water necessary to form a pourableaqueous calcined gypsum slurry on mixing with water and which is notaffected by exposure of the set composition to sunlight and which doesnot require a highly alkaline media for its effectiveness.

It is a further object of the invention to control the amount of waternecessary to form a pourable slurry of a calcined gypsum composition. Itis still another object of the invention to provide a method forpreparing cast gypsum articles using a less amount of water thannormally required and to provide a product prepared by the said methodhaving excellent strength and other physical properties includingresistance to discoloration on exposure to sunlight.

According to the invention, a composition for reducing the waternecessary to form a pourable slurry with powdered plaster compositionsis prepared by partially neutralizing a commercially availablecondensation product of napthalene sulfonic acid with potassium, therebeing present at least about 0.10% by weight and preferably about 20-30%by weight potassium as K₂ O based on the total weight of thecondensation product. Generally from about 1/2 to about 20 pounds ofsuch additive per ton (2,000 pounds) of dry plaster composition may beused in various different calcined gypsum compositions, although a rangeof about 5 to about 15 pounds per ton is preferred.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The naphthalene sulfonic acid and formaldehyde condensation products, orcondensed naphthalene sulfonates as they are sometimes known in thetrade, preferred in the present invention are anionic, polymer typedispersing agents supplied as light-colored powders or aqueous solutionshaving molecular weights varying between 300 and 3,000. They areprepared by condensing an alkylene or napthalene and an aldehyde such asformaldehyde with sulfonic acids derived from various petroleum crackingprocesses to yield water and the acid form which is generallyneutralized or partly neutralized with sodium hydroxide to give thesodium condensate salt as an article of commerce. The organic structureof the materials has not been completely determined, may be somewhatvariable, and does not appear to effect or concern the novel additivescontemplated herein. Condensed naphthalene sulfonates suitable for usein the present invention are marketed by a number of companies undervarious trade names. A number of naphthalene sulfonic acid andformaldehyde condensate materials are manufactured and marketed byDiamond Shamrock under the trademark "LOMAR" and their literature listschemical analysis and physical characteristics for a broad range ofmolecular sizes in the acid form or the sodium salt form. Othercondensed naphthalene sulfonates suitable for use in forming theadditive of the present invention are produced by W. R. Grace andCompany under the trademark "DAXAD" and their analysis and propertiesare produced in their technical literature.

The additives of the present invention are conveniently formed generallyby partly neutralizing any of the available condensed naphthalenesulfonates with a basic material containing potassium such as potassiumhydroxide. It does not appear necessary to effectuate completeneutralization or replacement of other salt forms with the potassiumforms; and it has been found completely satisfactory to merely providefor the presence of potassium in an amount from about 0.10 to 30% byweight of the condensate. Such may be accomplished by any convenientmeans such as dissolving potassium hydroxide or potassium sulfate intothe liquid acid form of the condensation product and drying andpulverizing the materials by any suitable means such as spray drying orconventional drying and grinding to form pulverulent materials or suchmeans such as solubilizing the available powdered salt condensednaphthalene sulfonate such as the sodium form with aqueous potassiumhydroxide followed by drying and grinding of the mixture of potassiumand condensate mixtures.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The plaster compositions may be based upon any conventional calcinedgypsum material including alpha calcium sulfate hemihydrate and betacalcium sulfate hemihydrate formed by various procedures. Such materialsmay be derived by calcination of natural gypsum rock or as a by-productof various chemical processes. The calcined gypsum compositions maycontain typical additives conventionally used in minor amounts to modifyproperties hereof for preferred commercial products.

The manner and order of incorporating the potassium form naphthalenesulfonate condensates of the invention with the calcined gypsum is notcritical. Thus the condensate may be mixed with the dry calcined gypsumor the already formed aqueous slurry of calcined gypsum. The condensatemay be added either separately or together with conventional additivesto the calcined gypsum in forming dry packaged industrial plastercompositions, or metered in powdered or water solubilized form into theaqueous slurry separately from or in combination with conventionaladditives during gypsum casting or molding operations.

In order to meet consistency and physical property requirements,particularly high density, high strength, and high surface hardnessrequirements necessary to the usage requirements in industrial molding,pottery and art plaster industries, many of the plaster compositions forindustrial casting are formulated with blends of beta calcium sulfatehemihydrate and alpha calcium sulfate hemihydrate, commonly referred toas "alpha gypsum". Alpha gypsum is known for its lower waterrequirements and resultant set cast higher density, higher strength, andhigher surface hardness set cast. However, it is also known for itsscarcity in production and commands a premium price because of specialproduction requirements necessary to obtain this form of calcinedgypsum.

In a series of evaluations, special industrial casting plastercomposition formulations were modified to substitute small amounts ofthe water reducing agent of the present invention or an unmodifiedcondensed naphthalene formaldehyde sulfonate into commercial products orthe plaster bases for such products then evaluated with respect tophysical properties, water absorption and cast discoloration.

EXAMPLE 1

In a first series of evaluations the commercially available LOMAR Dsodium salt form of condensed naphthalene sulfonate was treated topartially replace the sodium with potassium in determining consistencyreducing efficiency in a plaster composition of alpha calcined gypsumwithout the usual additives for commercial plaster formulations.

To incorporate the various amounts of potassium salt, 50 gram aliquotsof the commercial condensate LOMAR D, sodium salt form of naphthalenesulfonic acid and formaldehyde condensate which is a powder containingsodium equivalents of up to 11% Na₂ SO₄, were dissolved in 200 cc. ofdeionized water. The water was at room temperature, and the condensatewas completely solubilized in the water. Varying amounts of potassiumhydroxide or potassium sulfate were then added to the water solubilizedcondensate and thoroughly blended in a high speed mixer for 15 minutes.The resulting mixture was then dried at 250° F. to dryness and theresulting powder was ground to pass a 50 mesh (U.S. Standard) sieve. Thethoroughly dried and crushed mixture was added in various amounts to analpha calcined gypsum plaster base and evaluated for reduced waterrequirements (measured as normal consistency to achieve a pourableslurry) and setting time by standard hand set 300 gram Vicat set (ASTMC-472).

                  Table I                                                         ______________________________________                                                      Addition  Normal     Setting                                    Formulation   Level*    Consistency                                                                              Time                                       ______________________________________                                        Control A -                                                                   Alpha Gypsum  --        35-36 1/2  20-30                                      Alpha Gypsum                                                                  with Potassium                                                                Addition at:                                                                  #5 -5% KOH     5        26         17                                         #6 - 5% KOH   10        19.5       14                                         #7 - 5% KOH   15        19         20                                         #8 - 1% KOH    5        27.5       18                                         #9 - 1% KOH   10        21         13                                         #10 - 1% KOH  15        19         13                                         #11 - 0.5% KOH                                                                               5        27.5       13                                         #12 - 0.5% KOH                                                                              10        22         13                                         #13 - 0.5% KOH                                                                              15        19         10.5                                       #14 - 0.25% KOH                                                                              5        27.5       15                                         #15 - 0.25% KOH                                                                             10        22         13                                         #16 - 0.25% KOH                                                                             15        19         11                                         #17 - 5% K.sub.2 SO.sub.4                                                                    5        27.5       15                                         #18 - 5% K.sub.2 SO.sub.4                                                                   10        20.5       10                                         #19 - 5% K.sub.2 SO.sub.4                                                                   15        18          9                                         #20 - 1% K.sub.2 SO.sub.4                                                                    5        27.5       13.5                                       #21 - 1% K.sub.2 SO.sub.4                                                                   10        21.5       10                                         # 22 - 1% K.sub.2 SO.sub.4                                                                  15        18.5        9                                         #23 - 0.25% K.sub.2 SO.sub.4                                                                 5        27.5       13                                         #24 - 0.25% K.sub.2 SO.sub.4                                                                10        21.5        8                                         #25 - 0.25% K.sub.2 SO.sub.4                                                                15        18.5        7.5                                       ______________________________________                                         *lbs./ton (2,000 lbs.)                                                   

Typical results are set forth in Table I, reporting various amounts byweight of potassium hydroxide or potassium sulfate added to the aqueousmixture. From Table I it is quite clear that very little potassiumaugmentation need be provided to the sodium salt form of condensate inorder to achieve very substantial reductions in water requirements.Quite surprisingly, at each usage level of the additive of the inventionas set forth in Table I substantially the same water reduction effectwas obtained regardless of the amount of potassium provided forreplacement of sodium salt in the condensate.

EXAMPLE 2

In another series of evaluations a high potassium salt form of thecondensate of the invention was compared to the high sodium salt form ofcondensed naphthalene sulfonate using commercially available forms ofLOMAR D, the one form containing 22.5% potassium as K₂ O and the othercontaining 21% sodium as Na₂ O. They were evaluated in a low consistencyhigh strength commercial casting plaster containing alpha gypsum.

For the first evaluation the commercial plaster composition, Control B,was augmented with those amounts of the condensate salt addition aswould provide almost the same consistency reduction and evaluated forphysical properties, as set forth in Table II.

                  TABLE II                                                        ______________________________________                                                  Commercial                                                                    Alpha Gypsum                                                                  Composition                                                                            Plus Condensate                                                      (Control B)                                                                            Sodium     Potassium                                       ______________________________________                                        Additive     --        11 lbs./ton                                                                              5 lbs./ton                                  Normal                                                                        Consistency 35 cc.     27 cc.     28 cc.                                      Set Time    21 minutes 25 minutes 25 minutes                                  Time                                                                          of Temperature                                                                Rise After Set                                                                            28 minutes 34 minutes 36 minutes                                  Dry Cast Compres-                                                             sive Strength                                                                             7579 p.s.i.                                                                              11992 p.s.i.                                                                             12500 p.s.i.                                Dry Cast Density                                                              lbs./cubic foot                                                                           102.3      117.4      116.0                                       Two Week Slab                                                                 Discoloration                                                                  Wet        None       Considerable                                                                             Very Slight                                  Dry        None       Considerable                                                                             Very Slight                                 ______________________________________                                    

From Table II it can be seen that in order to achieve the sameconsistency results with the sodium form, the addition level had to beincreased to 11 pounds per ton compared to 5 pounds per ton for thepotassium form. This is a substantial jump in usage requirements inorder to obtain substantially equivalent performance.

In order to meet usage requirements in the industrial molding, potteryand art casting industry, plaster compositions must not only providehigh compressive strength but also must be resistant to sunlightdiscoloration, staining, and water absorption.

To evaluate for discoloration, the alpha calcined gypsum plaster base ofthe industrial casting plaster composition was evaluated by addingvarying amounts of the condensates to the plaster base and mixing withwater. The slurry was then formed into rectangular slabs measuring 3inches by 14 inches by 3/8 inch and exposed to sunlight for a two weekperiod of time. Some of the samples were placed immediately afterdemolding on a window ledge that received sunlight all day and reportedas "wet" results while duplicate samples were oven dried to constantweight before placing in sunlight and reported as "dry" results. Resultsare set forth in Table II as "two week slab discoloration" results. Thesodium form resulted in discoloration of the cast that would becommercially objectionable while the potassium form condensate of theinvention did not.

In another evaluation in this series, varying amounts of the condensatewere added to the plaster base of the gypsum composition and evaluatedfor water consistency reduction as set forth in Table III.

                  TABLE III                                                       ______________________________________                                                                         Consistency                                                                   Change From                                  Formulation                                                                              Usage Level                                                                              Consistency                                                                              Control                                      ______________________________________                                        Control C                                                                     (Plaster base                                                                 of Control B)                                                                            --         35 cc.     --                                           Sodium Salt                                                                   Condensate                                                                    (21% Na.sub.2 O)                                                                          5 lb./ton 31 cc.     -11%                                                    10 lb./ton 29 cc.     -17%                                                    15 lb./ton 27 cc.     -22%                                         Potassium Salt                                                                Condensate                                                                    (22.5% K.sub.2 O)                                                                         5 lb./ton 31 cc.     -11%                                                    10 lb./ton 23 cc.     -34%                                                    15 lb./ton 20 cc.     =42%                                         ______________________________________                                    

It may be clearly seen from Table III that the potassium condensate ofthe invention additive was much more effective in reducing waterrequirements at the same levels of usage as the sodium salt condensate.Each of the 10 and 15 pound per ton addition levels of the potassiumsalt condensate effectuated twice as much change in consistency over thecontrol as did the comparable sodium salt condensate. Further, it may beextrapolated from Tables II and IV that the sodium salt form at thehigher usage level would be commercially objectionable for providingdiscoloration of cast articles.

In another evaluation in this series two different high strength lowconsistency industrial casting plaster formulations containing alphagypsum were compared to a plaster of Paris plaster compositioncontaining customary additives and further including either thepreferred potassium condensate additive of the invention or its mostcomparable sodium salt form condensate, LOMAR D condensate and thesamples evaluated for physical properties as set forth in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    Formulation                                                                            Control           Control                                            Ingredients                                                                             D     1     2     E     3     4                                     __________________________________________________________________________    Commercial                                                                    Industrial                                                                    Plaster  100%   --    --   100%   --    --                                    Plaster                                                                       Of Paris                                                                      Composition                                                                             --   99.92%                                                                              99.92%                                                                               --   99.94%                                                                              99.94%                                 Condensate -                                                                  Sodium Salt                                                                   (21% Na.sub.2 O                                                               assay)    --   .08%   --    --   .06%   --                                    Condensate -                                                                  Potassium Salt                                                                (22% K.sub.2 O                                                                assay)    --    --   .08%   --    --   .06%                                   Properties                                                                    Normal                                                                        Consistency                                                                   cc       59    59    59    62    60    60                                     Set Time -                                                                    Minutes  22    22    19    21    17    16                                     Temp. Rise.sup.1                                                                       17    17    19    25    17    18                                     Dry Cast                                                                      Compressive                                                                   Strength psi                                                                           2,998 2,802 2,723 2,888 2,820 2,762                                  Dry Cast                                                                      Density                                                                       lb/cu ft 72.5  72.2  71.6  72.9  72.7  72.3                                   Setting                                                                       Expansion                                                                              0.174%                                                                              0.187%                                                                              0.216%                                                                              0.238%                                                                              0.196%                                                                              0.227%                                 Ring Test.sup.2                                                               Water                                                                         Absorption                                                                    cc/min.  2.75  4.90  2.65   --    --    --                                    Weight Gain.sup.3                                                             at 5 min.                                                                              22.5% 24.1% 21.3%                                                     10 min. 23.8% 26.1% 22.9%                                                     15 min. 24.1% 27.4% 24.3%                                                     30 min. 25.4% 28.7% 25.6%                                                     1 hour  26.4% 29.1% 26.3%                                                    Wet-Out/Dry-                                                                  Out.sup.4                                                                     % wt. loss                                                                             3.9%  3.8%  4.0%                                                     __________________________________________________________________________                Control D  1          2                                           __________________________________________________________________________    Two Week Slab                                                                 Discoloration                                                                  Wet        None       Considerable                                                                             Slight                                       Dry        None       Considerable                                                                             Very slight                                 __________________________________________________________________________     .sup.1 Temperature rise at: the time in minutes to reach the maximum          temperature during the exothermic hydration of calcined gypsum setting to     hydrated gypsum is indicative of the point in the reaction at which almos     all of the plaster has rehydrated and the sample has obtained its maximum     wet strength. Determined by casting a sample to a breadth of 2 inches in      container as described in ASTM C472 and portionate with the thermocouple      in an insulated cabinet then measuring the time during which the              temperature rises.                                                            .sup.2 Ring test: simulates water absorption characteristics to give an       indication of dewatering rate. The plaster is cast into a 2 inch cube         shape, oven dried to constant weight and cooled to room temperature in a      sealed bag to prevent moisture pickup. Then placed on a flat surface          within a 1 inch inside diameter by 1 inch high aluminum ring secured to       the surface with a rope of modeling clay. Water (10 cc.) is poured into       the ring and timed for absorption into the cast sample. If leakage under      the ring is observed, the test is discontinued. The absorption rate into      the sample is obtained by dividing 10 cc. by the elapsed time for the         water to absorb into the cast sample and the value reported is the averag     of duplicate evaluations.                                                     .sup.3 Weight gain: Compares relative absorption rates of samples when 2      inch cubes of the sample materials are fully immersed in water for 1 hour     Prior to immersion, samples are dried to constant weight, cooled to room      temperature in a sealed plastic bag, then weighed to a 1 gram accuracy.       Thereafter the samples are placed in an appropriate sized container to be     completely covered with water throughout the 1 hour period of time. At 5      minute intervals, the sample is removed from the water, lightly blotted       with a towel and weighed to the nearest gram, then returned to the water.     Absorption rate as percentally gained is determined by subtracting the        initial dry weight from the wet weight at the timed interval, dividing by     the initial dry weight and multiplying by 100.                                .sup.4 Wet-out dryout tests were conducted on oven dried 2 inch cubes by      weighing the dried cubes, fully immersing them in water for 4 hours,          reweighing them, then drying the samples for 20 hours in a 110° F.     oven and reweighing again. This procedure is continued for a 2week period     to determine if the formulations eroded at comparable rates to simulate       manner in which they may erode in pottery shop usage.                    

From Table IV it may be clearly seen that the potassium additiveprovided greater resistance to discoloration and less water absorptionin plaster compositions for industrial casting while maintaining otherphysical properties at acceptable levels.

EXAMPLE 3

As set forth in Tables III and IV above additives may be included intheir conventional amounts for their usual purposes, such asaccelerators, retarders, defoamers and the like. Other adjuvants in moremajor amounts may also be added without deliterious affects in the useof the additives of this invention. For example, Portland cement may beadded in varying amounts without deleterious affects. In fact thereappears to be an enhanced consistency efficiency with the condensate ofthe invention, at least at low levels of Portland cement augmentation tocalcined gypsum plaster of Paris compositions, as set forth in Table V.

                  TABLE V                                                         ______________________________________                                                  Alpha Gypsum                                                                  Plus 5% Port-                                                                          Additive Addition Level                                    Formulation:                                                                              land Cement                                                                              Sodium     Potassium                                   Properties  (Control E)                                                                              11 lb./ton 5 lb./ton                                   ______________________________________                                        Normal Consis-                                                                tency (cc.) 40         27         28                                          Setting Time                                                                  (Minutes)   20         25         25                                          Time of maximum                                                               Temperature                                                                   Rise After Set                                                                (minutes)   20         26         19                                          Dry Density                                                                   (lbs./cubic ft.)                                                                          --          123       125.7                                       Dry Compressive                                                               Strength    6125       12,217     13,866                                      Set Expansion                                                                             0.250%      .255%      2.77%                                      Two Week Slab                                                                 Discoloration                                                                  Wet        None       Considerable                                                                             None                                         Dry        None       Considerable                                                                             None                                        ______________________________________                                    

What is claimed is:
 1. A plaster composition consisting essentially ofcalcined gypsum and a minor amount of a water-reducing agent for thecalcined gypsum, the water-reducing agent comprising a potassium salt ofnaphthalene sulfonic acid condensate having a molecular weight betweenabout 300 and 3,000, said condensate containing from about 0.10% toabout 30% by weight of potassium expressed as potassium oxide.
 2. Theplaster composition of claim 1 in which the condensate is naphthalenesulfonic acid formaldehyde condensate and contains about 0.10% to about30% by weight of the condensate of potassium expressed as K₂ O.
 3. Theplaster composition of claim 1 wherein the calcined gypsum is alphacalcined gypsum calcium sulfate hemihydrate.
 4. The plaster compositionof claim 1 wherein the calcined gypsum is beta calcium sulfatehemihydrate.
 5. The plaster composition of claim 1 wherein thewater-reducing agent is present in the plaster composition in an amountof from about 1/2 pound to about 20 pounds per ton.
 6. The plastercomposition of claim 1 wherein the water-reducing agent is present inthe plaster composition in an amount of about 5 pounds to about 15pounds per ton.
 7. The plaster composition of claim 1 wherein thepotassium salt contains about 20% to about 25% potassium expressed aspotassium oxide.
 8. The plaster composition of claim 1 wherein thecondensate is naphthalene sulfonic acid formaldehyde condensatecontaining about 10% to about 30% by weight of potassium expressed as K₂O and the condensate is present in the plaster composition in an amountof from about 5 pounds to about 15 pounds per ton.
 9. A method ofreducing the amount of water required to be mixed with a dry plastercomposition to obtain a pourable slurry which comprises adding to acalcined gypsum plaster composition from about 1/2 pound to about 30pounds per ton of napthalene sulfonic acid condensate having a molecularweight from about 300 to about 3,000 and containing by weight of thecondensate from about 10% to about 30% of potassium expressed aspotassium oxide.
 10. The method of claim 9 wherein the condensate isadded to a dry plaster composition.
 11. The method of claim 9 whereinthe condensate is added to an aqueous slurry of calcined gypsum.
 12. Themethod of claim 9 wherein the condensate contains about 20 to about 25%by weight of potassium expressed as potassium oxide, and the condensateis added in an amount of about 5 to about 15 pounds per ton of drycalcined gypsum plaster.
 13. The method of claim 9 wherein thecondensate is naphthalene sulfonic acid formaldehyde condensatecontaining about 10% to about 30% by weight of potassium expressed as K₂O and the condensate is added in an amount of about 5 pounds to about 15pounds per ton.